Field of the Invention
The present invention relates to a work vehicle and a control method for the work vehicle.
Background Information
Recently, a hybrid-type work vehicle has been proposed that travels using both an engine and a motor. The above-mentioned hybrid-type power transmission device demonstrates the advantage of having an energy recovery function and good fuel consumption in comparison to a conventional power transmission device (referred to below as a “torque converter-type speed variator”) having a torque converter and a multi-stage speed variator. A hydraulic-mechanical transmission (HMT) or an electric-mechanical transmission (EMT) is disclosed as the hybrid-type power transmission device for work vehicles as in, for example, Japanese Laid-Open Patent Publication No. 2006-329244.
The HMT includes a planetary gear mechanism, and a first pump/motor and a second pump/motor connected to a rotating element of the planetary gear mechanism. The first pump/motor and the second pump/motor function as either hydraulic motors or hydraulic pumps in response to the travel state of the work vehicle. The HMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the pump/motors.
An electric motor is used in the EMT in place of the hydraulic motor in the HMT. Specifically, the EMT includes a first generator/motor and a second generator/motor. The first and second generator/motors function as either electric motors or electric generators in response to the travel state of the work vehicle. Similar to the HMT, the EMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the generator/motors.
The hybrid-type work vehicle typically generates electricity with the electric motor during low loads or during braking, stores the electricity in an energy storage unit such as a capacitor/battery and the like, and uses the electrical power of the energy storage unit to drive the electric motor during high loads. Consequently, the work vehicle is able to reduce fuel consumption with the engine. Japanese Laid-Open Patent Publication No. 2002-359935 discloses a hybrid-type work vehicle that appropriately changes the range of appropriate electricity storage amounts from the potential energy of the work implement and from the kinetic energy of a revolving body, whereby the electricity storage amount does not exceed an upper or lower limit even when the capacity of the energy storage unit is small. Based on a maximum value and a minimum value of the electricity storage amount in the energy storage unit, the upper and lower limits are set to as to be provided with a margin so that overcharging and overdischarging do not occur even when overshooting or undershooting occurs due to the control of the electricity storage amount.